The present invention relates to article surveillance and more particularly to article surveillance systems generally referred to as of the magnetic type.
In 1934 a French Pat. No. 763,681, was granted to Pierre Picard for a "Method for Locating Objects by Modifying a Magnetic Field." At the heart of the Picard system was the recognition that different samples of metallic material produced different harmonic signals when detected by an "electrodynamic balance," which different signals could be used to recognize one sample as distinct from another. The patent observes that a piece of copper will produce only a fundamental frequency component, while a piece of iron will produce a signal containing, in addition to the fundamental term, a certain number of harmonics. On the other hand, it also observes that metals with an initially very high permeability, such as mu metal or permalloy or permafy, also furnish harmonics, and the label, i.e., harmonic number, of these harmonics is much higher than in the case of iron. Therefore, by incorporating a suitable filter to detect a particular harmonic it is possible to recognize the presence of such high permeability material. As an example, Picard describes recognizing a piece of permalloy by detecting a 650 hertz component, i.e., the 13th harmonic, when the exciting field has a frequency of 50 hertz. At another point in the patent, Picard indicates that the third harmonic at 150 hertz is preferably employed.
Subsequent to Picard, a long list of patents have been issued on inventions seeking to improve upon the selectivity and reliability of systems intended to detect magnetic markers. These patents emphasize selecting suitable geometry coupled with low coercive force and high permeability in order to provide a distinctive and detectable signal. An extensive summary of these patents relating to magnetic marker detection will be found in Richardson, U.S. Pat. No. 4,222,517, issued Sept. 16, 1980. Said patent refers to a typical interrogation field as having a peak amplitude of approximately 1 Gauss varying at a frequency between 60 Hz and 10 kHz. The magnetic strip is identified as permalloy that has been annealed for maximum response with final coercivity of from 0.01 to 0.1 oersteds and a permeability on the order of 200,000 Gauss/Oersted. The dimensions of the strip are given as typically 3 inches long, 0.0007 inches thick and 0.125 inches wide. However, it is stated that the thickness and width can vary plus or minus 20 percent while the length can range from less than 2 inches to as much as 7 inches. Finally, Richardson asserts that the most accepted system design detects the 18th to 20th order harmonics of the fundamental interrogation frequency.
The Richardson patent contains extensive reference to U.S. Pat. No. 3,765,007, issued Oct. 9, 1973 to James T. Elder. In its "summary of the invention" section, the Elder patent describes the nature of the system contemplated for use with the markers described therein. In the words of the patent, the system comprises "equipment for applying in the [interrogation] zone a periodically varying magnetic field which increases at a predetermined time rate of change." The significance of this statement will become apparent during the subsequent description of the present invention.
For the marker, the Elder summary states, inter alia, that it may take the form of a thin, flat ferromagnetic ribbon or wire having a magnetic moment of at least 0.1 electromagnetic unit, while the ratio of the length to the square root of the cross-sectional area should be at least 150. This is stated as ensuring that self-demagnetizing field effects do not increase the switching field beyond 20 oersteds. It is also stated that "conductive wire markers should have a diameter of 10 to 300 microns." However, it should be noted that Elder describes his preferred embodiment as consisting of open-strip sections of "an annealed permalloy ribbon . . . about 25 microns thick, 18 centimeters long and 0.6 centimeter wide."
Also of background interest is the disclosure found in Montean U.S. Pat. No. 4,075,618, issued Feb. 21, 1978. In column 5 of that patent, commencing in line 3, the patentee considers the types of materials from which his marker can be constructed. For the purpose of producing "high order harmonics" (in excess of the twentieth order) the active portion of the marker is preferably formed of "very high permeability material such as Permalloy . . . having a coercivity of not greater than 0.5 oersted, and preferably having coercive forces in the range of 0.02 oersted." The patent continues with the recitation that "the actual permeability of such a material is desirably in the range of 10.sup.6," and goes on to identify other suitable materials, to wit: "Supermalloy, `METGLAS`, an amorphous metallic alloy having low coercive force and high permeability, manufactured by the Allied Chemical Company, and `Mark II Permalloy` such as the manufactured by Carpenter Technology, Inc." This section of the patent concludes with the statement that "Permalloy which has been annealed after it has been fabricated into the desired shapes to further enhance the permeability may be particularly desired."
With the exception of "METGLAS", all of the materials mentioned in the previously identified prior patents have been crystalline. An additional discussion of the use of non-crystalline, i.e., amorphous, metal for article surveillance markers will be found an Gregor et al. U.S. Pat. No. 4,298,862, issued Nov. 3, 1981. The Gregor et al. patent includes a recitation that an amorphous marker is prepared by cooling a melt of the desired composition at a rate of at least about 10.sup.5 degrees C./sec employing well known quenching techniques.
All of the magnetic markers described in the prior art mentioned above have in common the fact that they cause a detectable perturbation to an incident magnetic field in the process of reversing magnetic polarity. When such material is driven around its hysteresis loop, particularly from one polarity to the opposite, a signal pulse is produced. The shape of this pulse is a function of the time it takes to reverse polarity, i.e., proceed from one saturation point to the other, or from a residual induction point to the reverse saturation point. This time element is a function of the time rate of change of the incident field between levels sufficient to effect such polarity reversal. Hence, the statement found in the Elder patent and quoted above.
All available evidence reveals consistent and continuous endeavor to find materials with higher and higher permeability and lower and lower coercivity. The object of such endeavor has been to find markers that produce high order harmonics with sufficient amplitude to be readily detectable. With the same object in view, systems have been designed to operate at relatively high frequencies and/or with strong incident fields, and the latter has been attained generally by establishing narrow surveillance zones to limit the distance from marker to antenna.
In spite of the many years of effort obviously devoted to the problem, and some 50 years after Picard, none of the markers heretofore known has been able to produce in response to a surveillance field interrogation a signal sufficiently unique that the marker is free from being mimicked by at least some commonplace article. For example, certain samples of nickel plating have been known to produce signals containing harmonic components that cause false alarms in systems designed to detect permalloy markers.
It is with the above in mind that it is an object of the present invention to provide a marker having a unique characteristic enabling it to be detected without fear of false alarms from any presently known commonplace article.
It is a further object of the present invention to provide a surveillance marker whose response to an interrogating field is essentially independent of the time rate of change of the incident field.
A further object of the present invention is to provide such marker whose response is substantially independent of the incident field strength so long as such strength exceeds some low threshold level.
Another object of the present invention is to provide a surveillance marker with a magnetically interrogatable element that has a unique response characteristic notwithstanding that such element does not rely upon having high permeability and low coercivity. A corollary to this objective is that such unique response is readily obtainable with sufficient amplitude to be detected readily.